A magnetic disk storage apparatus (hereinafter sometimes simply referred to as a disk drive) typified by a hard disk drive (HDD) comprises a servo system for positioning a head at a target position on a disk. The servo system demodulates a servo pattern (servo data) recorded in a servo area provided on the disk to demodulate (calculate) a position signal (position information) indicative of the position of the head on the disk.
The servo pattern is roughly classified into a null servo pattern and an area servo pattern. The null servo pattern comprises two burst fields with different magnetization patterns. Thus, a disk drive using the null servo pattern can have a reduced servo area compared to a disk drive using the area servo pattern, and advantageously ensures a larger data area.
The servo system carries out servo signal demodulation comprising demodulating a null servo pattern from a readback signal read by a reader (or a read head) and correcting the resultant demodulation signal to demodulate a position error signal (PES) indicative of the position of the head (reader). For the servo signal demodulation, a linearity correction that corrects an error between a demodulated position and an actual position is particularly important in demodulating a position error signal accurately indicative of the position of the reader. In the linearity correction, a calculation of an optimal correction coefficient for the correction functions is important.
Correction functions using correction coefficients have been proposed, and methods for determining the optimal correction coefficient for a correction function have been proposed. However, the conventional methods take much processing time to determine the optimal correction coefficient and may be susceptible to measurement errors when the amplitude of the null servo pattern is measured. This may make the linearity correction insufficient or excessive, hindering implementation of servo signal demodulation that demodulates an accurate position error signal.